Cyclomaltodextrin glucanotransferase (E.C. 2.4.1.19), also designated cyclodextrin glucanotransferase or cyclodextrin glycosyltransferase, in the following termed CGTase, catalyses the conversion of starch and similar substrates into cyclomaltodextrins via an intramolecular transglycosylation reaction, thereby forming cyclomaltodextrins, in the following termed cyclodextrins (or CD), of various sizes. Commercially most important are cyclodextrins of 6, 7 and 8 glucose units, which are termed .alpha.-, .beta.- and .gamma.-cyclodextrins, respectively. Commercially less important are cyclodextrins of 9, 10, and 11 glucose units, which are termed .delta.-, .epsilon.-, and .zeta.-cyclodextrins, respectively.
Cyclodextrins are thus cyclic glucose oligomers with a hydrophobic internal cavity. They are able to form inclusion complexes with many small hydrophobic molecules in aqueous solutions, resulting in changes in physical properties, e.g. increased solubility and stability and decreased chemical reactivity and volatility. Cyclodextrins find applications particularly in the food, cosmetic, chemical and pharmaceutical industries.
Most CGTases have both starch-degrading activity and transglycosylation activity. Although some CGTases produce mainly .alpha.-cyclodextrins and some CGTases produce mainly .beta.-cyclodextrins, CGTases usually form a mixture of .alpha.-, .beta.- and .gamma.-cyclodextrins. Selective precipitation steps with organic solvents may be used for the isolation of separate .alpha.-, .beta.- and .gamma.-cyclodextrins. To avoid expensive and environmentally harmful procedures, the availability of CGTases capable of producing an increased ratio of one particular type of cyclodextrin is desirable.
CGTases from different bacterial sources, including CGTases obtained from Bacillus, Brevibacterium, Clostridium, Corynebacterium, Klebsiella, Micrococcus, Thermoanaerobacter and Thermoanaerobacterium have been described in the literature.
Thus Kimura et al. [Kimura K, Kataoka S, Ishii Y, Takano T and Yamane K; J. Bacteriol. 1987 169 4399-4402] describe a Bacillus sp. 1011 CGTase, Kaneko et al. [Kaneko T, Hamamoto T and Horikoshi K; J. Gen. Microbiol. 1988 134 97-105] describe a Bacillus sp. Strain 38-2 CGTase, Kaneko et al. [Kaneko T, Song K B, Hamamoto T, Kudo T and Horikoshi K; J. Gen. Microbiol. 1989 135 3447-3457] describe a Bacillus sp. Strain 17-1 CGTase, Itkor et al. [Itkor P, Tsukagoshi N and Udaka S; Biochem. Biophys. Res. Commun. 1990 166 630-636] describe a Bacillus sp. B1018 CGTase, Schmid et al. [Schmid G, Englbrecht A, Schmid D; Proceedings of the Fourth International Symposium on Cyclodextrins (Huber O, Szejtli J, Eds.), 1988 71-76] describe a Bacillus sp. 1-1 CGTase, Kitamoto et al. [Kitamoto N, Kimura T, Kito Y, Ohmiya K; J. Ferment. Bioeng. 1992 74 345-351] describe a Bacillus sp. KC201 CGTase, Sakai et al. [Sakai S, Kubota M, Nakada T, Torigoe K, Ando O and Sugimoto T; J. Jpn. Soc. Starch. Sci. 1987 34 140-147] describe a Bacillus stearothermophilis CGTase and a Bacillus macerans CGTase, Takano et al. [Takano T, Fukuda M, Monma M, Kobayashi S, Kainuma K and Yamane K; J. Bacteriol. 1986 166 (3) 1118-1122] describe a Bacillus macerans CGTase, Sin et al. [Sin K A, Nakamura A, Kobayashi K, Masaki H and Uozumi T; Appl. Microbiol. Biotechnol. 1991 35 600-605] describe a Bacillus ohbensis CGTase, Nitschke et al. [Nitschke L, Heeger K, Bender H and Schultz G; Appl. Microbiol. Biotechnol. 1990 33 542-546] describe a Bacillus circulans CGTase, Hill et al. [Hill D E, Aldape R and Rozzell J D; Nucleic Acids Res. 1990 18 199] describe a Bacillus licheniformis CGTase, Tomita et al. [Tomita K, Kaneda M, Kawamura K and Nakanishi K; J. Ferm. Bioeng. 1993 75 (2) 89-92] describe a Bacillus autolyticus CGTase, Jamuna et al. [Jamuna R, Saswathi N, Sheela R and Ramakrishna S V; Appl. Biochem. Biotechnol. 1993 43 163-176] describe a Bacillus cereus CGTase, Akimaru et al. [Akimaru K, Yagi T and Yamamoto S; J. Ferm. Bioeng. 1991 71 (5) 322-328] describe a Bacillus coagulans CGTase, Schmid G [Schmid G; New Trends in Cyclodextrins and Derivatives (Duchene D, Ed.), Editions de Sante, Paris, 1991, 25-54] describes a Bacillus firmus CGTase, Abelian et al. [Abelian V A, Adamian M O, Abelian L A A, Balayan A M and Afrikian E K; Biochememistry (Moscow) 1995 60 (6) 665-669] describe a Bacillus halophilus CGTase, and Kato et al. [Kato T and Horikoshi K; J. Jpn. Soc. Starch Sci. 1986 33 (2) 137-143] describe a Bacillus subtilis CGTase.
EP 614971 describes a Brevibacterium CGTase, Haeckel & Bahl [Haeckel K, Bahl H; FEMS Microbiol. Lett. 1989 60 333-338] describe Clostridium thermosulfurogenes CGTase, Podkovyrov & Zeikus [Podkovyrov S M, Zeikus J G; J. Bacteriol. 1992 174 5400-5405] describe a Clostridium thermohydrosulfricum CGTase, JP 7000183 describes a Corynebacterium CGTase, Binder et al. [Binder F, Huber O and Bock A; Gene 1986 47 269-277] describe a Klebsiella pneumoniae CGTase, U.S. Pat. No. 4,317,881 describes a Micrococcus CGTase, and Wind et al. [Wind R D, Liebl W, Buitelaar R M, Penninga D, Spreinat A, Dijkhuizen L, Bahl H; Appl. Environ. Microbiol. 1995 61 (4) 1257-1265] describe Thermoanaerobacterium thermosulfurigenes CGTase.
A CGTase produced by Thermoanaerobacter sp. has been reported by Norman & Jorgensen [Norman B E, Jorgensen S T; Denpun Kagaku 1992 39 99-106, and WO 89/03421], however, its amino acid sequence has never been disclosed. Here we report the nucleotide sequence encoding the Thermoanaerobacter sp. CGTase (presented as SEQ ID:NO 1), as well as its amino acid sequence (presented as SEQ ID:NO 2).
Also, CGTases from thermophilic Actinomycetes have been reported [Abelian V A, Afyan K B, Avakian Z G, Melkumyan A G and Afrikian E G; Biochemistry (Moscow) 1995 60 (10) 1223-1229].
Recently protein engineering has been employed in order to modify certain CGTases to selectively produce more or less of a specific cyclodextrin.
The Structure of CGTases
CGTases are functionally related to .alpha.-amylases. CGTases and .alpha.-amylases both degrade starch by hydrolysis of the .alpha.-(1,4)-glycosidic bonds, but produce virtually exclusively cyclic and linear products, respectively.
Members of the CGTase family possess a high overall amino acid sequence identity, more than 60%. CGTases and .alpha.-amylases share about 30% amino acid sequence identity. However, the active site clefts of CGTases and .alpha.-amylases, located between the A and B domain (Asp229, Glu257 and Asp328), are rather similar.
Recently, the tertiary structures of CGTases were determined. Thus, Hofman et al. [Hofman B E, Bender H, Schultz G E; J. Mol. Biol. 1989 209 793-800] and Klein & Schulz [Klein C, Schulz G E; J. Mol. Biol. 1991 217 737-750] report the tertiary structure of a CGTase derived from Bacillus circulans Strain 8, Kubota et al. [Kubota M, Matsuura Y, Sakai S and Katsube Y; Denpun Kagaku 1991 38 141-146] report the tertiary structure of a CGTase derived from Bacillus stearothermophilus TC-91, Lawson et al. [Lawson C L, van Monifort R, Strokopytov B, Rozeboom H J, Kalk K H, de Vries G E, Penninga D, Dijkhuizen L, and Dijkstra B W; J. Mol. Biol. 1994 236 590-600] report the tertiary structure of a CGTase derived from Bacillus circulans Strain 251, Strokopytov et al. [Strokopytov B, Penninga D, Rozeboom H J, Kalk K H, Dijkhuizen L and Dijkstra B W; Biochemistry 1995 34 2234-2240] report the tertiary structure of a CGTase derived from Bacillus circulans Strain 251, which CGTase has been complexed with acarbose, an effective CGTase inhibitor, and Knegtel et al. [Knegtel R M A, Wind R D, Rozeboom H J, Kalk K H, Buitelaar R M, Dijkhuizen L and Dijkstra B W; J. Mol. Biol. 1996 256 611-622] report the tertiary structure of a CGTase derived from Thermoanaerobacterium thermosulfurigenes.
These and other studies reveal that Bacillus circulans CGTases are composed of five domains. The three-dimensional structures also reveal that the N-terminal domains of CGTases have structural similarities to those of .alpha.-amylases, whereas the C-terminal domains were found to be unique to CGTases.
The catalytic site of CGTases is located in the A domain, and has three catalytic residues (in Bacillus circulans strain 251 these are Asp229, Glu257 and Asp328, respectively, cf. Strokopytov et al. 1995, op cit.). A central amino acid residue is located in the B domain, around which residue the cyclodextrins are formed, i.e. the cyclization axis. Substitution of this central residue, e.g. tyrosine at residue 188 in Bacillus ohbensis (corresponding to position 195, CGTase numbering) in order to increase the relative production of .gamma.-cyclodextrin to .beta.-cyclodextrin has been the object of the study described by Sin et al. [Sin K, Nakamura A, Masaki H, Matsuura Y and Uozumi T; Journal of Biotechnology 1994 32 283-288] and JP-A-5219948.
Nakamura et al. [Nakamura A, Haga K and Yamane K; Biochemistry 1994 33 9929-9936] describe the effects on substrate binding and cyclization characteristics by replacements carried out at four residues in the active center of a Bacillus sp. Strain 1011 CGTase. In these CGTase variants, a phenylalanine at position 183 has been replaced by leucine, a tyrosine at position 195 has been replaced by alanine, phenylalanine, leucine, threonine, valine, and tryptophan, respectively, a phenylalanine at position 259 has been replaced by leucine, and a phenylalanine at position 283 has been replaced by leucine.
Penninga et al. [Penninga D, Strokopytov B, Rozeboom H J, Lawson C L, Dijkstra B W, Bergsma J and Dijkhuizen L; Biochemistry 1995 34 3368-3376] describe the effect on activity and product selectivity of site-directed mutations in tyrosine at position 195 of a Bacillus circulans Strain 251 CGTase. In this publication four CGTase variants have been produced, in which variants the tyrosine at position 195 have been replaced by phenylalanine, tryptophan, leucine and glycine, respectively.
Fujiware et al. [Fujiwara S, Kakihara H, Sakaguchi K and Imanaka T; J. Bacteriol. 1992 174 (22) 7478-7481] describe CGTase variants derived from Bacillus stearothermophilus, in which a tyrosine residue at position 191 (corresponding to position 195 CGTase numbering) has been replaced by phenylalanine, a tryptophan residue at position 254 (corresponding to position 258, CGTase numbering) has been replaced by valine, a phenylalanine at position 255 (corresponding to position 259, CGTase numbering) has been replaced by phenylalanine and isoleucine, respectively, a threonine residue at position 591 (corresponding to position 598, CGTase numbering) has been replaced by phenylalanine, and a tryptophan residue at position 629 (corresponding to position 636, CGTase numbering) has been replaced by phenylalanine.
JP-A-7023781 describes CGTase variants derived from Bacillus sp. 1011, in which a tyrosine residue at position 195 has been replaced by leucine, valine, phenylalanine and isoleucine, respectively.
JP-A-5244945 describes CGTase variants derived from Bacillus stearothermophilus TC-91, in which tyrosine residues at positions 222 and 286 (corresponding to positions 195 and 259, CGTase numbering) have been replaced by phenylalanine in order to increase the relative production of .alpha.-cyclodextrin to .beta.-cyclodextrin.
JP-A-5041985 describes CGTase variants derived from Bacillus sp. #1011, in which histidine at residue 140 in region A, histidine at residue 233 in region B, and histidine at residue 327 in region C, respectively, have been replaced by arginine and asparagine residues, respectively.
EP 630,967 describes CGTase variants in which a tyrosine residue at position 211 of a Bacillus sp. 290-3 CGTase (corresponding to position 195, CGTase numbering), at position 217 of a Bacillus sp. 1-1 CGTase (corresponding to position 195, CGTase numbering), and at position 229 of a Bacillus circulans CGTase (corresponding to position 195, CGTase numbering), have been substituted for tryptophan and serine.
Up to now, all efforts in making CGTase variants have lead to substitutions in the region around the active site, in particular at the central cyclization residue, corresponding to position 195, CGTase numbering. Only few CGTase variants holding substitutions at more distant regions have been suggested, and the manufacture of these variants have not been based on any particular concept.